The invention relates to electrostatic actuators, and more particularly, to electrostatic actuators with intrinsic stress gradient.
Electrostatically actuated beams are one of the fundamental building blocks in Micro-Electro-Mechanical System (MEMS) devices, and find applications in a variety of fields, such as communications, sensing, optics, micro-fluidics, and measurement of materials properties. In the field of communications, electrostatically actuated MEMS variable capacitors and RF switches are used in tunable RF filter circuits. The electrostatically actuated MEMS capacitors and RF switches offer several advantages over solid state varactor diodes, including a more linear response and a higher quality (Q) factor. For example, MEMS-tuned High Temperature Superconductor (HTS) resonators employing electrostatically actuated capacitors have demonstrated a frequency tuning range of about 7.5% with a Q factor above 2000 at a frequency of 850 MHz and a temperature of 77xc2x0 K.
A conventional electrostatically actuated MEMS capacitor comprises a fixed bottom electrode on a substrate, and a flexible cantilever beam that acts as a top movable electrode of the capacitor. The cantilever beam is secured at one end to the substrate by a rigid anchor. The capacitance of the electrostatically actuated capacitor is changed by applying a bias voltage to the capacitor. The applied voltage establishes an electrostatic force on the beam that deflects the cantilever beam, thereby changing the inter-electrode gap between the top electrode (cantilever beam) and the bottom electrode, which in turn changes the capacitance. Therefore, the change in the inter-electrode gap of the capacitor upon electrostatic actuation of the cantilever beam changes the capacitance of the capacitor. The initial position of the cantilever beam with no applied bias voltage establishes the low (or off) capacitance of the capacitor. A curved cantilever beam allows for a lower capacitance in the off state (as compared to a straight one), which is desirable for many applications. A common approach for setting the initial curvature (with no applied bias) of the cantilever beam is to construct the beam using a layered stack of two metals with different Coefficients of Thermal Expansion (CTE). A beam constructed from two metals is often referred to as a bimetallic beam. The CTE mismatch between the two metals in the bimetallic beam produces a stress gradient in the beam that causes the beam to curve as the temperature is changed. In order to achieve an initial upward curvature of the bimetallic beam at cryo-temperatures, the metal with the higher CTE (e.g., aluminum) is deposited on top of the metal with the lower CTE (e.g., gold).
Although the usefulness of conventional electrostatically actuated devices has been demonstrated, there are a number of problems with these devices stemming from the use of the bimetallic beam. The mechanical properties of the bimetallic beam have been shown to change with time due to inter-metallic diffusion between the two metals and recrystallization, thereby limiting the long term reliability and reproducibility of these devices. The introduction of a diffusion barrier layer between the two metal layers can help reduce inter-metallic diffusion but can not solve it completely. Furthermore, the operation of these devices requires precise temperature control because any variation in temperature leads to a change in the deflection of the bimetallic beam, which in turn changes the capacitance of these devices.
Ideally the two metals used to construct the bimetallic beam are ductile, resistant to fatigue or work hardening, electrical conductive, and easy to process. However, the requirement that the two metals have different CTEs in order to achieve a desired initial curvature restricts the types of metal that are available to construct the bimetallic beam. Typically, a less than ideal metal (e.g., indium, aluminum or zinc) has to be used for one of the two metals of the bimetallic beam in order to achieve a desired initial curvature. As a result, the mechanical and electrical properties of the cantilever beam are compromised.
The present invention provides an electrostatic actuator with intrinsic stress gradient. The electrostatic actuator according to the invention uses a metal layer made substantially of a single metal with an intrinsic stress gradient induced therein instead of a bimetallic beam.
An electrostatic actuator, built in accordance with an embodiment of the invention, comprises an electrode and an electrostatically actuated beam fixed at one end relative to the electrode. The electrostatically actuated beam comprises a metal layer made substantially of a single metal with a stress gradient induced therein. The stress gradient in the metal layer causes the beam to curve away from the electrode. Upon electrostatic actuation of the beam, the beam is deflected from its initial position relative to the electrode.
An electrostatically actuated variable capacitor, built in accordance with another embodiment of the invention, comprises a substrate, a bottom electrode on the substrate, a curved cantilever beam acting as a top movable electrode of the capacitor, and a rigid anchor for securing one end of the cantilever beam to the substrate. The cantilever beam comprises a metal layer made substantially of a single metal with a stress gradient induced therein. The stress gradient in the metal layer causes the beam to initially curve upwardly from the substrate. Upon electrostatic actuation of the beam, the beam is deflected from its initial curvature relative to the bottom electrode, thereby changing the intro-electrode gap between the top electrode (cantilever beam) and the bottom electrode and the capacitance of the capacitor.
In one embodiment of the invention, the stress gradient is induced in the metal layer of the beam by varying process conditions during deposition of the metal layer.
In another embodiment of the invention, the metal layer is deposited using electroplating. In this embodiment, the stress gradient is induced in the metal layer of the beam by varying the electroplating bath, current density, and/or temperature during electroplate deposition of the metal layer.
In another embodiment, the metal layer is deposited in a two step deposition process of a soft metal deposition followed by a hard metal deposition, where the deposited hard metal exhibits higher intrinsic stress than the soft metal. The soft and hard metal are both made of the same metal, e.g., gold, characterized by different crystal grain sizes. In one embodiment, the electroplating bath used to deposit the hard metal contains dopants to enhance the intrinsic stress in the hard metal.
In another embodiment of the invention, the metal layer is deposited using evaporation, such as electron beam evaporation. In this embodiment, the stress gradient is induced in the metal layer by varying the deposition rate during deposition of the metal layer. In one embodiment, an initial upward curvature of the beam is obtained by increasing the deposition rate during deposition of the metal layer.
In another embodiment of the invention, the metal layer is deposited using sputtering. In this embodiment, the stress gradient is induced in the metal layer by varying the gas pressure of the sputtering during deposition of the metal layer. In one embodiment, an initial upward curvature of the beam is obtained by increasing the gas pressure of the sputtering during deposition of the metal layer.
In another embodiment of the invention, the cantilever beam is fully hinged to the anchor at one end.
In another embodiment of the invention, the cantilever beam is double hinged to the anchor at one end.
In another embodiment of the invention, the front edge of the cantilever beam is straight.
In another embodiment of the invention, the front edge of the beam is curved to alleviate stress concentration due to sharp corners.